Every morning on the giant exoplanet WASP-94A b, clouds made from rocky minerals gather across the sky. By evening, those clouds are gone.
Using observations from the James Webb Space Telescope (JWST), astronomers discovered this dramatic daily weather cycle on the distant world, located nearly 700 light years from Earth in the constellation Microscopium. The findings mark one of the first times scientists have directly observed cloud cycling on a Hot Jupiter exoplanet.
The discovery also gave researchers a much clearer view of the planet's atmosphere, helping them better understand what the world is made of and how its weather behaves. The study was published in the journal Science.
"I've been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side. We've known for quite a while that clouds are pervasive on Hot Jupiter planets, which is annoying because it's like trying to look at the planet through a foggy window," said co-author and program PI, David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins. "Not only have we been able to clear the view, but we can finally pin down what the clouds are made out of and how they're condensing and evaporating as they move around the planet."
Extreme Weather on WASP-94A b
To study WASP-94A b, scientists observed the planet as it crossed in front of its host star. During this transit, JWST was able to separately examine the leading and trailing edges of the planet as it moved across the star's light.
The leading edge represents the planet's morning side, where atmospheric winds carry air from the cooler night side toward the intensely hot day side. The trailing edge acts as the evening side, where air moves back toward darkness.
The observations revealed a striking difference between morning and evening conditions. The morning side was packed with clouds made of magnesium silicate, a mineral commonly found in rocks on Earth. The evening side, however, appeared almost cloud free.
Researchers believe there are two possible explanations for the disappearing clouds. One idea is that powerful winds drag the clouds deep into the planet's atmosphere on the scorching day side, effectively hiding them from view. Another possibility is that the clouds evaporate as they move into temperatures exceeding 1,000 degrees, similar to morning fog burning away on Earth but under far more extreme conditions.
"It was a huge surprise. People have expected some differences, like its cooler in the morning than the evening -- that's something natural that we experience here on Earth," Sing said. "But what we saw was a real dichotomy between the weather on both sides of the planet, and huge differences in cloud coverage, and that changes our whole picture of the planet."
James Webb Peers Through Alien Clouds
The clearer evening skies gave scientists an opportunity that had previously been impossible with older telescopes such as Hubble. By isolating the cloud free side of the planet, researchers could directly examine the atmosphere itself instead of averaging cloudy and clear regions together.
"With the Hubble telescope, when we used to do this type of observation, we got an average view of the whole planet with data from the clouds and the atmosphere squished together and indistinguishable," said first author Sagnick Mukherjee, a postdoctoral fellow at Arizona State University who was a student at Johns Hopkins and UC Santa Cruz at the time of the research. "This approach with the JWST lets us localize our observations, which helped us see the cloud cycle."
The clearer data also solved a long standing mystery about the planet's chemistry. Earlier measurements suggested WASP-94A b contained hundreds of times more oxygen and carbon than Jupiter, something that did not fit with existing theories of planet formation.
The new observations paint a very different picture. Scientists now estimate the planet contains only about five times more oxygen and carbon than Jupiter, making it far more similar to the giant planet in our own solar system than previously believed.
A New Window Into Alien Atmospheres
Hot Jupiters are giant gas planets that orbit extremely close to their stars, even closer than Mercury orbits the Sun. Because of their intense heat and radiation, these planets provide scientists with ideal natural laboratories for studying atmospheric chemistry and cloud behavior under extreme conditions.
After studying WASP-94A b, the research team examined eight additional Hot Jupiters and identified similar cloud cycling on two more worlds: WASP-39 b and WASP-17 b.
Next, researchers plan to expand the search using a larger JWST observing program that will investigate cloud cycles across many different exoplanets, including an unusual gas giant that travels through the habitable zone on an eccentric orbit.
